To use a CD-RW you'll either need to erase the disk then copy all your info back to it with the new stuff, or do a multisesion burn. On the PC side if you mean you used to use your CD's like floppy disks, then you were probabily using a software program called Direct CD that comes with EZ CD Creator. Personally I hate this program as if you creat a disk using it, the only computers that can read that disk are ones that have Direct CD on them. As Far as I know there is no equivilant program for the Mac and I hope there never is.

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